The performance of a high-speed fiber-optic link depends very strongly on the efficiency with which light is modulated by the input electrical signal. This modulation efficiency is often referred to as the “slope efficiency” of the modulation. See, for example, C. Cox, Analog Optical Links: Theory and Practice, Cambridge University Press, 2004. In links that use a Mach-Zehnder interferometric modulator as the modulation device, the slope efficiency depends directly on a parameter known as Vπ. The parameter Vπ is relatively difficult to measure using prior art methods and apparatus.